1. Field of the Invention
This invention relates generally to in-circuit testing apparatus and more particularly to an improved in-circuit test apparatus for printed circuit boards having an improved alignment and vacuum system for reliable contact with the circuit boards and utilizing sensitive spring probes for interfacing with a circuit analyzer.
2. Description of the Prior Art
In the field of printed circuit card testing, it is common to isolate and verify the individual components or test points in an integrated circuit board with a test connector. After each part is individually tested and proven functional, the printed circuit board is deemed to operate properly.
In the 1950s, the General Electric Company introduced the first in-circuit testing system. This system, referred to as "Guarding", was an electronic means designed to analyze the individual components in an integrated electrical circuit. In this test mode, multiple probes connected to a circuit analyzer on one end were tied to all the test points of a printed circuit card at the other end. Thus, "Guarding" resulted in increased visibility of the circuit card under test. Later, improvements were developed in analyzers manufactured by Systemation, Fairchild, and Zhentel.
In order to accomplish this method of testing, fixturing coupled with "Guarding" formed the basis for in-circuit testing. These fixtures generally included spring probes projecting out of the top of the test fixture and the circuit board under test was set on top of the fixture. The fixture was designed such that a space existed between the bottom and top of the fixture and an apparatus for drawing a vacuum in that space was connected thereto. Then, a vacuum was applied to the circuit board through the penetrations for the spring probes in the top part drawing the circuit board down against and making electrical contact with the spring probes. The spring probes were electrically connected by circuit wiring to an in-circuit electronic analyzer which performed a circuit analysis upon the vacuum held circuit board. Various problems have existed in the in-circuit test fixtures as herein described.
In order for the probes to make contact with the proper test points of the circuit board, the bottom stationary plate and the top plate supporting the circuit board under test must remain in a parallel relationship. Prior attempts to accomplish this requirement included utilizing cold-rolled steel pins with guide bushings. In the manufacturing environment, one machinist would drill the bottom plates while another machinist would drill the top plates. The result was that the holes failed to properly align and when the steel pins were inserted, a nonbinding fit was not achieved. To hold the circuit board to the top plate, a vacuum was applied resulting in the warping of the top panel, loss of vacuum seal, and misdirection of the spring probes onto the circuit board.
A second problem was that the prior art utilized a vacuum sealant system between the stationary bottom plate and the top plate that supported the circuit board. Such sealing system utilized square corners which made holding the vacuum between the plates difficult. For example, the Fairchild vacuum seal was a flat, hard rubber wiping seal that once it became activated, in addition to having square corners, resulted in vacuum leakage and poor electrical contact between the spring probes and the circuit board under test.
Because each test probe must be electrically insulated from all other probes to obtain reliable test data, the component parts of the test fixture must be constructed of an electrical insulating material. Thus, prior attempts to satisfy this condition led to the use of plastic parts and the machining of alignment holes therein. Due to the chemical composition of plastics, environmental conditions such as humidity, heat, and cold result in the warping and shrinkage of the plastic and the ultimate misalignment of the machine drilled holes.
A further problem with the previous designs, especially the Fairchild "Thinline" vacuum fixture, involved the spring probe access holes in the top component and the vacuum seal located between the circuit board under test and the top component. Referring to the problem involving the spring probe access holes, the top and bottom plate components are drilled separately in the production environment. Thus, the tolerances caused the guide system and guide pins to misalign and typically there was no method to adjust or compensate for drilling error. Also, if the plates were fabricated from plastic, the previously described material distortion could also cause misalignment. Therefore, the access hole for each spring probe had to be large to compensate for the high tolerance guide system to prevent damage to the probes when the component plates collapsed upon one another under vacuum. In particular, when test points on the circuit board were close together, the spring probes projecting from the bottom plate component were closely spaced. Therefore, large access holes had to be drilled adjacent to one another and consequently the chain of holes became a continuous hole in the top component accommodating many probes. This resulted in limited protection for and likelihood of damage to each individual probe. The second component of this problem involved a vacuum seal that must exist to retain vacuum between the circuit board under test and the top component to hold the circuit board in position. Normally, this vacuum is achieved by the same vacuum between the top and bottom plate components through the spring probe access holes previously described. In order to prevent the loss of this vacuum, a peripheral seal was designed to be placed between the circuit board and the top plate. This peripheral seal was usually comprised of a thick foam rubber material as in the Fairchild "Thinline" model. When initially fitted, the thick foam rubber seal caused uneven distribution of vacuum pull on the circuit board resulting in the bowing of the circuit board. Such distortion caused the probes contacting the circuit board center to make excessive contact while the probes contacting the circuit board periphery made insufficient contact. Also, the thick foam rubber would tend to depress under vacuum pressure and become contaminated causing the seal to leak and reduce the probe-circuit board contact even further.
In addition to vacuum leakage resulting from poor seals, the problem of intermittent or total loss of vacuum plagued the prior art. Because of misalignment and warping of the component plates when under vacuum, the plates would physically touch one another and either temporarily or permanently eliminate the vacuum. This action resulted in chattering of the two plates preventing continuous electrical contact between the probes and the circuit board under test. Attempts to solve this problem by the Fairchild Company resulted in utilizing a sheet of fiberglass or phoenolic material as a top plate and gluing rubber stops directly to the bottom plate. The purpose of the rubber stops were to eliminate the loss of vacuum by maintaining a separation distance between the component plates. As with the previously described plastic component plates, environmental factors caused the rubber stops to wear and loose adhesion to the bottom plate.
Finally, maintenance access to the interior of the test fixture of the prior art required the removal of multiple screws or bolts from the plate components. Thus, in order to make any adjustments to the spring probes, the vacuum seal or the vacuum connection, the test fixture had to be removed from service. This required the tedious disassembly of the test fixture by maintenance personnel to remedy the situation.